(a) Field of the Invention
The present invention relates generally to an apparatus for the non-invasive wide-band measurement of high frequency currents. More particularly, it relates to an apparatus which surrounds a conductor and measures the high frequency currents in the conductor by transducing the magnetic field produced by the currents.
(b) Description of Related Art
Non-invasive current measurements are commonly made using a split-core transformer assembly that can be opened and then clamped around a current carrying power line. These clamp-on ammeters typically consist of a ferrous core wrapped with a secondary winding. When clamped around a power line, changing currents in the power line or primary circuit produce a changing magnetic flux that is coupled through the core to the secondary winding. The coupled flux produces a voltage across the secondary winding that is proportional to the rate of change in primary current. Thus, clamp-on ammeters provide a simple, non-invasive apparatus for measuring current in a conductor by deriving the primary current from the secondary voltage. Clamp-on ammeters are frequently used to measure the power consumption of large electric motors in commercial and industrial applications. Their applications are limited, however, because they are optimized to operate at power line frequencies of 50 Hz or 60 Hz, and their accuracy decreases significantly for small variations in the power line frequency.
The measurement of high frequency currents has become an increasing concern because humans are being subjected to an increasing number of electromagnetic radiators. Of particular concern are industrial workers who may be exposed to large doses of electromagnetic radiation from radio frequency heat sealers, inductive heaters, and high voltage power transmission lines. Another area of special concern includes cancer patients undergoing therapeutic treatments, such as diathermy and hyperthermia, that apply localized high frequency electromagnetic energy directly to the patient""s body. These treatments, although promising, are difficult to apply with precision because present application methods provide poor feedback to the physician regarding the dose and aberrant heating within the patient""s body. Aberrant heating can, and often does, cause significant damage to the patient""s body.
Thus, there is a strong commercial need for a non-invasive device that can accurately measure a broad range of high frequency currents. More specifically, there is a considerable need for such a device that is readily adaptable for use in measuring these high frequency currents within the human body.
Traditionally, the non-invasive measurement of high frequency currents is accomplished using the same principle employed by clamp-on ammeters. To operate effectively, high frequency ammeters or current probes use special transformer geometries, materials, and construction. Typically, high frequency current probes use ferrous cores wrapped with a low resistance secondary winding. These types of high frequency current probes offer excellent sensitivity to the magnetic fields generated by high frequency currents, but they suffer from several serious drawbacks. The ferrous core increases the inductance of the primary circuit (i.e., the conductor being monitored), and the magnitude of the this inductance increases as the conductor (e.g., a human limb) occupies a larger fraction of the probe aperture. Additionally, the ferrous core has a high permeability coefficient that efficiently couples impedances in the secondary circuit, such as the winding resistance and the input impedance of the meter connected to the secondary winding (e.g., a 50xcexa9 input), into the primary circuit. These changes in the primary circuit impedance due to the presence of the current probe are commonly referred to as insertion impedances. Insertion impedances are highly undesirable because they change the primary current that would normally be flowing in the absence of the current probe. As a result, measurement errors can be significant, particularly in applications requiring the measurement of low-level magnetic fields. Additionally, ferrous cores have a permeability coefficient that varies with frequency, they are heavy and inflexible, they must have a small aperture size to be practicable, they are subject to magnetic saturation, and they are expensive. Thus, ferrous core current probes are not readily adaptable for use in measuring high frequency currents in the limbs and other portions of a human body.
Many drawbacks inherent in current probes with ferrous cores can be overcome by using a construction based on a non-ferrous core. Non-ferrous cores may be made from a variety of plastics, or may be a hollow form that supports the secondary winding over a core consisting primarily of air. Such non-ferrous cores do not substantially disturb the primary currents they are measuring because their insertion impedance is much smaller than that of ferrous core types. Furthermore, non-ferrous cores have a permeability coefficient (xcexco) that does not vary with frequency, they are not subject to magnetic saturation, they can be constructed to form lightweight, flexible loops with a large aperture size, and they are inexpensive. Thus, non-ferrous core current probes are readily adaptable for accurately measuring high frequency currents in a human body.
Although non-ferrous core current probes show great promise in the measurement of high frequency currents within the human body, they continue to suffer from several practical problems. First, the secondary winding of such non-ferrous core probes must incorporate series or shunt resistance to prevent resonances that would otherwise cause extreme variations in sensitivity as a function of frequency. This resistive loading of the secondary reduces the sensitivity and signal-to-noise ratio of the current probe. The sensitivity is also reduced because of the lower permeability of the core. Furthermore, because the voltage output is proportional to the rate of change of the magnetic flux through the secondary winding, the ratio of the output voltage to the primary current is proportional to the frequency. The combined effects of resistive loading, low permeability, and the fundamental frequency dependance of sensitivity significantly limit the useful frequency bandwidth of these probes. In addition, the lower sensitivity and poor signal-to-noise ratio of non-ferrous current probes present difficulties in accurately measuring the low-level magnetic fields produced by the high frequency currents passing through a human body. These limitations both impair and limit the number of viable commercial applications. In practice, existing non-ferrous current probes are limited to applications having fundamental frequencies above 100 MHz. This limited bandwidth prevents their application in several commercial applications such as RF heat sealers that expose factory operators to 27.12 MHz radiation.
In accordance with one aspect of the present invention, an apparatus for measuring high frequency currents includes a non-ferrous core current probe that is coupled to a wide-band transimpedance amplifier. The current probe has a secondary winding with a winding resistance that is substantially smaller than the reactance of the winding so that the sensitivity of the current probe is substantially flat over a wide band of frequencies.